Hendre-ddu quarry is located on the eastern slopes of Craig-y-garn, to the east of Garndolbenmaen. This was originally a small early nineteenth-century open working with material being directly carted away. It developed into a pit with tunnel access to a small mill and an incline down to the road in Cwm Pennant. As work progressed requiring lower adits, a turbine powered mill, possibly of wood, was built at the foot of the incline. In 1861 60 men produced 700 tons per annum. In 1898 just five men were employed here.The quarry is portrayed as 'Disused' on the Ordnance Survey 25-inch plan of 1900.
Remains in 1991 were fairly extensive. These included, at the original highest working level, barracks, dressing shed and a weigh-house; below, a mill and a weighbridge; the incline with several adits leading onto it; powder house and traces of the roadside mill.

Sources:
Ordnance Survey County Series 25-inch map: sheet Caernarvonshire XXVII.14, editions of 1889 & 1900.
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), p.99.

David Leighton, RCAHMW, 12 February 2015